The invention relates to a method and arrangement for the surge protection of inverters for photovoltaic systems, comprising at least one surge protection device which is physically integrated in the inverter or located in the vicinity thereof, wherein the at least one surge protection device is connected to the direct current side of the inverter, according to patent claims 1 and 7.
In typical photovoltaic systems several photovoltaic modules are connected in series to form a photovoltaic string so as to obtain a direct voltage of some 100 to 1000 V, respectively 1500 V, suited for the direct voltage-alternating voltage (DC-AC) conversion and the subsequent feeding into the mains.
The photovoltaic string in question is connected either directly to the inverter, or several strings, the configuration of which is as identical as possible, are connected in parallel and connected to a central inverter.
The construction of photovoltaic energy systems is regulated in DIN VDE 0100-712 (VDE 0100-712). Corresponding systems may comprise one or more sub-generator(s).
Relevant for the safe operation of DC-AC inverters is, among others, a sufficient surge protection for the electronic components of the inverter including the EMC protection circuits located there.
Photovoltaic inverters have a broad MPP (Maximum Power Point) voltage range, e.g. type-dependent between 320 and 800 V, with a maximum direct voltage of 1000 V, respectively a minimum direct voltage of approximately 150 V.
Depending on the respective MPP voltage internal current paths are switched in the inverter. To this end, so-called boost converters or buck converters are used.
It is known from the DE 40 32 569 C2 document to provide each photovoltaic module with an integrated inverter with MPP tracking. Such an MPP tracker regulates the voltage to that value at which the system works at maximum power, i.e. in the MPP range. To this end, the MPP tracker varies the drawn current by a defined amount, calculates the instantaneous power and readjusts the current value to a higher power. By means of a control unit signals are transmitted to a data bus which supplies these data to the power and control part for checking the operational capability of the module.
The DE 199 04 561 C1 document describes a method and a circuit arrangement for the maximum power point control of solar generators. According to the solution described in this document a sensor is provided which is electrically insulated from the solar generator. The current characteristic of the sensor is determined quasi-continuously in a manner known per se. The performance characteristic is then calculated from the current characteristic. A controlled variable for the converter is derived quasi-continuously from the maximum. With regard to such a circuit arrangement it is assumed that the sensor used is a solar module which is of the same type as the solar modules used in the solar generator and that a measurement of the characteristics and a microcomputer for detecting the controlled variable of the controller are switched in between the sensor and the converter control.
If solar generators of this type are connected to inverters subjected to a maximum power point control advantages are obtained with respect to the efficiency of the system as a whole. Different MPP voltage values, i.e. voltage values resulting in the greatest power output, involve difficulties with respect to the necessary surge protection of a corresponding inverter, however, because surge protection devices, e.g. varistors, suppressor diodes or spark gaps, each have a defined response voltage, respectively response voltage range, on account of the construction and action thereof. For instance, if the surge protection is set at a maximum direct voltage of 1000 V the protective function in an MPP voltage range of, for instance, 500 V is only insufficient.